Hi Isaac, if you haven't been to the Smokies, it's definitely a must-do if you live in the SE. It's also a great place for someone to have their first backpacking trip. Dependent upon where in the park you are, the shelters are nice for newbies if really bad weather hits, all the trails are signed and pretty clearly marked, the campsites have bear poles or food lockers to make protecting food easy, etc.
I don't have maps handy, so I'm not positive of mileage, but I have a route that's a favorite of mine. It would be perfect for you and your brother, provided you are both in shape and prepared to ascend a bit. Start from the Cosby access, ascend Rattlesnake Den Ridge (passing Albright Grove, one of the best old-growth stands in the East) to the AT. South on the AT to Tricorner Knob, then the Laurel Mtn. Trail to Laurel Gap, Mt. Sterling Ridge trail, descend Mt Sterling on either Swallow Fork or Baxter Creek. Ascend Low Gap to the AT, then return to Cosby using either the Mt. Cammerer trail, or Low Gap.
This route takes in two of the big drainages in Cosby Creek and Big Creek, old-growth timber, fishing and swimming on Big Creek, and the ridge between Tricorner and Mt. Sterling is my favorite place in the park. About 10-12 miles of high-elevation (at least for the SE) ridge walking with amazing views of Big Creek and Cataloochee.
The big concern would be rain. I've been on Mt. Sterling's summit 25 times or so, and I've had two clear days that I recall. Bears are a concern, but general bear awareness is all that's required to be safe. I wouldn't worry about canisters or spray, although if bear spray makes you or your brother feel more comfortable, it's very little weight to trade for peace of mind. It seems most of the problem bears are south of Newfound Gap, although Cosby has the worst bear incident on record in the park. Remember though that if you have a problem with bears in the backcountry, it probably has as much to do with your practices as the bear. They are curious critters, so don't do things to arouse their curiosity!
Have fun planning! Get your brother involved with the planning as well; trip-planning gives newbies some ownership of their experience, rather than feeling like they're just tagging along. There are so many resources available, but the first thing to do is go to GSMNP's site and download the trail map. It's an easy way to look at the layout of trails and think about potential routes and how they'll connect. See you on the trails Brotha!